WATCH: Sea turtles come ashore to nest by the thousands at Costa Rica’s Ostional beach

OSTIONAL, Guanacaste — Just before the waves start to crest, their heads break the water’s surface as the creatures bob, waiting for a chance to come ashore. A wave crashes on the beach of the Ostional Wildlife Refuge as the sun sets, and as the water retreats, it reveals a sea turtle.

On Dec. 4, thousands of olive ridley sea turtles (Lepidochelys olivacea) came ashore in the latest arribada, a mass nesting of the turtles that lasts several days. That night, Pablo Chavarría, with Costa Rica’s National Park Service, estimated that 19,000 turtles came ashore. During a typical arribada as many as 300,000 sea turtles arrive at the beach over several days.

The female turtles haul themselves out of the sea with just their flippers, raising their heads above the wet sand occasionally to exhale deeply, almost as if they were sighing with the effort.

Once they’ve reached a suitable site, the turtles use their back flippers to excavate the nest, leaving behind roughly 100 leathery eggs, each about the size of a ping-pong ball. After the eggs are in the sand the turtles cover up the nest. For good measure, the turtles rock back and forth, making loud thumping sounds as they “dance” over the nest to compact the sand and camouflage it.

This arribada was a smaller affair than in September when some 5,000 tourists descended on the beach town to see it. The handful of local guides and two National Conservation officers were no match for the crowds who rivaled the turtles in numbers that afternoon.

Today, the rules are much stricter. If visitors want to stand among the sea turtles they need to be accompanied by a local guide and travel in groups of nine or so in shifts. Travelers can get a local guide on the main (and only) road through Ostional at a small green cinderblock building leading toward the beach. Sunset is one of the most picturesque times to see the turtles come out of the water, but guides take visitors on night tours too, using red lights to show the throngs of turtles digging their nests.

Residents pay $3 and foreign visitors pay $8 each; a small price for a once-in-a-liftime experience.

But timing is everything, as the saying goes, and arribadas are no different. Olive ridley turtles come ashore approximately once a month during the year but they come in the largest numbers during Costa Rica’s rainy season between June and October. Visitors need to stay in Ostional — which has simple cabins for rent near the beach — or in the larger tourist destinations of Sámara, Nosara or Tamarindo if they want to be close enough to take advantage of their good timing.